The present invention relates to a method of and a system for detecting errors in harmonizing the optical axis of an optical instrument relative to a reference direction, in particular the required sighting direction. It also relates to an optical instrument including a detector system of the above kind.
Remote sensing satellites and recent space telescopes include many instruments which must be pointed simultaneously in the same reference direction, which is accurately determined by means of star sighting systems.
In an attempt to eliminate humanization errors between the optical axes of said instruments and the reference direction, which errors are essentially due to vibrations and temperature variations, the following three actions have been used individually or in combination:
moving said optical components closer together,
fixing said optical instruments to a rigid structure which is thermally very stable, and
applying thermal control to said structure.
Also, it is essential to check the harmonization obtained in this way, if possible periodically.
The measurements effected by such instruments are subject to errors caused by stray light, and bulky diaphragms or screens are disposed at the light entry of said instruments in an attempt to eliminate such errors.
The object of the present invention is to remedy the above drawbacks and improve the accuracy of sighting optical instruments relative to a reference direction (given by a star sighting system, for example), to improve the efficiency with which said instruments reject stray light, and to make the instrument insensitive to mechanical and thermal distortion.
To this end, the method of the invention for detecting errors in harmonizing the optical axis of an optical instrument with a reference direction is noteworthy in that:
said reference direction is materialized by a reference light beam; and
said optical instrument incorporates:
an optoelectronic detector disposed in the focal plane of said optical instrument;
an a focal optical system coaxial with the optical axis of said optical instrument and which has a real entry pupil and forms a real image thereof; and
an invariant optical axis translation system disposed in the vicinity of said real image of the entry pupil, said invariant system receiving said reference light beam and, regardless of the angle of incidence of said beam, generating an exit light beam which is parallel to said reference beam and which is directed towards said optoelectronic detector from at least approximately the center of said real image of the entry pupil, said optoelectronic detector delivering an electrical signal representative of the eccentricity of the light spot formed by said exit beam in said focal plane relative to the point at which the optical axis of said instrument passes through said optical plane.
Accordingly, the system of the present invention for detecting said harmonization errors includes:
means for generating a reference light beam materializing said reference direction; and
incorporated in said optical instrument:
an optoelectronic detector disposed in the focal plane of said optical instrument;
an a focal optical system coaxial with the optical axis of said optical instrument and which has a real entry pupil and forms a real image thereof; and
an invariant optical axis translation system disposed in the vicinity of said real image of the entry pupil, said invariant system receiving said reference light beam and, regardless of the angle of incidence of said beam, generating an exit light beam which is parallel to said reference beam and which is directed towards said optoelectronic detector from at least approximately the center of said real image of the entry pupil.
Thus, by virtue of the present invention:
real entry and exit pupils are created by means of said a focal optical system, which reduces stray light and consequently the volume of the optical instruments, which no longer require any baffles in the form of screens and/or diaphragms. It has been confirmed that, in the case of a star sighting system, the present invention reduces stray light by a factor of the order of 1000 compared to a prior art instrument with screens and/or diaphragms of equivalent bulk;
said output light beam can be injected at the center of said real image of the entry pupil, so that there is no resulting parallax error and it is therefore possible to inject a slightly defocused reference beam; and
a light spot is formed on said optoelectronic detector in the focal plane of said optical instrument by said exit light beam so that the position of that light spot on said optoelectronic detector relative to the point at which the optical axis of said instrument passes through the latter detector is at all times representative of the instantaneous error in harmonizing the axis of said optical instrument relative to said reference direction.
Thus an electronic signal which is representative of said harmonization error is available at the output of said optoelectronic detector, which is of the CCD type, for example. The harmonization error signal can therefore be used for dynamic correction of pointing errors of said optical instrument. To this end, although it would be feasible to use said harmonization error signal to slave the position of the optical axis of said instrument to said reference direction, it is preferable to implement it, for example in software, to correct the image of the scene observed by said optical instrument by re-centering it in said exit light beam.
As is known in the art, the image of the scene observed by the optical instrument and formed by said optoelectronic detector takes the form of a succession of frames. Thus, in order to obtain a particularly distinct harmonization error signal at the output of said optoelectronic detector, a pulsed reference light beam is used and:
either a reference light beam pulse is directed onto said optoelectronic detector during some image frames, referred to as first image frames, and not during other image frames, referred to as second image frames, for example during one frame in two, and a second image frame is subtracted from a first image frame each time, so that the difference image comprises only the trace of said exit light beam, such subtraction eliminating the scene observed by the optical instrument and noise. This leaves only the error in harmonizing the axis of the optical apparatus and the reference direction;
or very short light pulses, for example pulses 1000 times shorter than the image frames, are directed onto said optoelectronic detector, said pulses being interleaved between the image frames. Thus observation of the scene observed by the optical instrument is not disturbed and the rejection of background noise from said scene is good.
The reference light beam can be of any kind and collimated or not. Note that, given chromatic aberrations of the optics of the instrument, the wavelength selected for the reference beam enables a slightly defocused image spot to be obtained, if necessary.
In a first embodiment of the invention, the cross-section of said exit beam is of the same order of magnitude as the area of the sensitive face of each of the individual photosensitive components constituting said optoelectronic detector and said optoelectronic detector has the structure of a matrix in which said photosensitive components are disposed in rows and columns. In this case in particular it is especially advantageous if said reference beam is a laser beam.
Alternatively, the cross-section of said exit beam is large compared to the area of the sensitive face of each of the individual photosensitive components constituting said optoelectronic detector, said exit beam forms the image of a test pattern in said focal plane and said optoelectronic detector includes at least one strip of such photosensitive components.
It is particularly advantageous if said test pattern has at least two rectilinear sections inclined relative to each other and relative to said strip.
Said a focal optical system can be implemented with lenses and/or concave mirrors, in a manner known in the art. It is advantageous for its magnification K to be greater than 1, for example equal to 3, because this reduces the constraints of implementing said invariant system by the factor K.
Said invariant optical axis translation system advantageously includes three reflective surfaces on the inside faces of a rectangular trihedron forming a hollow cube corner, two of said reflective surfaces are disposed to receive said reference light beam and the third of said reflective surfaces is intersected by the optical axis of said instrument and is disposed to direct said exit light beam onto said optoelectronic detector.
It is therefore possible to offset said optical instrument relative to said reference light beam with no loss of pointing accuracy.
To enable said optoelectronic detector to receive the scene observed by said instrument and said reference light beam equally well, said third reflector surface takes the form of a parallel-sided plate, through which the light rays from said scene pass. Note that said plate can be small in size and accommodated in a thermally protected location inside said instrument without increasing its overall size.
Said parallel-sided plate is preferably treated to increase its reflectivity for said reference light beam.
For reasons of overall size in the vicinity of said optical instrument, it can be advantageous for said means for generating said reference light beam to be remote from said optical instrument, for example at a distance of several meters. This spacing is made possible by the structure of the system according to the present invention.
The present invention also relates to an optical instrument whose optical axis must be harmonized with a reference direction, the instrument including a harmonization error detector system as described hereinabove.
Note also that, by virtue of the present invention:
it is possible to reduce dimensional constraints of mechanical and thermal origin when harmonizing a plurality of sighting axes;
the accuracy of pointing one or more optical axes can be improved, which is particularly advantageous in the case of star sighting systems;
the detector system is insensitive to defects of positioning of said invariant optical axis translation system;
the telescope of an interferometer array (aperture synthesis) can be aligned with the various channels of a power laser provided that said reference light beam is split into a plurality of reference beams; and
inertial and gyroscopic systems and gyro-lasers can be recalibrated by star sighting, possibly by effecting a plurality of axis translations.